1. Field of the Invention
As the configuration of load devices connected to power facilities installed in factories and buildings is diversified in configuration, they become more sensitive to drops in power supply stability such as a power supply interruption and an abnormal voltage drop. However, there is an increasing need to prevent the shutdown of the load device due to drops in the power supply stability and to thereby preclude inconveniences to the community. To receive a reliable power supply, consumers of electricity adopt a two-circuit feeding system so that, for example, they can switch from one circuit to the other if one circuit fails. The present invention relates to a power supply system for a load which, in order to supply power to the load from a plurality of different alternating current power sources, selectively switches to any of the plurality of power sources to supply power to the load. More specifically, the present invention relates to an electric power supply system for a load which, in the event of an abnormality such as a voltage irregularity or an interruption of power in one power circuit, can switch the load, without any interruption, to another power circuit in normal operating conditions.
2. Description of the Related Art
FIG. 5 shows a conventional power supply system for load. Referring to FIG. 5, there are shown an alternating current power source system 1 (hereinafter referred to as a first alternating current power source system), another alternating current power source system 2 (hereinafter referred to as a second alternating current power source system), a mechanical-type switch 3 (hereinafter referred to as a first switch) for switching the first alternating current power source system 1, a voltage transformer (PT) 31 for detecting the voltage at the first alternating current power source system 1, a mechanical-type switch 4 (hereinafter referred to as a second switch) for switching the second alternating current power source system 2, another voltage transformer 41 for detecting the voltage at the second alternating current power source system 2, and a load 5, for example, a mechanical apparatus, a lighting system, an air-conditioning system, an elevator system, or a computer or a controller for controlling these systems and the like. A switch controller 342 receives the outputs of the voltage transformers 31, 32, and controls the first switch 3 and second switch 4 to selectively turn them on and off in response to the outputs of the voltage transformers 31, 32. When the second alternating current power source system 1 is in normal operating conditions, the switch controller 342 causes the first switch 2 to close while opening the second switch 3 so that power from the first alternating current power source system 1 is fed to the load 5. When a power interruption takes place in the first alternating current power source system 1, power can not be continuously supplied to the load, and the switch controller 342 causes the second switch 4 to close after opening the first switch 2 so that the second alternating current power source system 2 feeds power to the load 5 via the second switch 4. Hence, the load 5 is continuously supplied with power. Since the first switch 3 and second switch 4 are mechanical-type switches, the changeover from the first alternating current power source system 1 to the second alternating current power source system 2 takes several hundred ms or more, and during the changeover time, an interruption of power to the load is unavoidable. The above-described power supply system or power receiving system, employing the mechanical-type switches, works with a load capable of tolerating a transient interruption of power, but when the load 5 is a computerized apparatus or equipment, which is sensitive to a transient interruption of power or an abnormal voltage drop, such a system cannot be used. When the load 5, such as a computerized apparatus or equipment, is sensitive to a transient power interruption or abnormal voltage drop, the following conventional technique is employed.
FIG. 6 shows another conventional example employing such a conventional technique, namely, a power supply switching system that includes thyristor switches instead of the mechanical-type switches to assure fast changeover speed in the changeover operation of power supply systems. Referring to FIG. 6, there are shown a first thyristor switch 6 composed of a pair of thyristors in anti-parallel connection, and a second thyristor switch 7 composed of a pair of thyristors in anti-parallel connection. When a first alternating current power source system 1 is in normal operating conditions, the thyristor pair of the first thyristor switch 6 are continuously supplied with a gate signal to keep them in a closed state and the thyristor pair of the second thyristor switch 7 are not supplied with a gate signal to keep them in an open state. Thus, the first alternating current power source system 1 thus feeds power to the load 5 via the first thyristor switch 6. When a power interruption takes place in the first alternating current power source system 1, the gate signal to the thyristor pair of the first thyristor switch 6 is immediately stopped, and the first thyristor switch 6 is opened at the moment a current flowing through the thyristor reaches zero. The thyristor pair of the second thyristor switch 7 are then supplied with a gate signal to turn them on, and a second alternating current power source system 2 feeds power to the load 5 via the second thyristor switch 7. When the thyristor switches are used, the changeover from the first alternating current power source system 1 to the second alternating current power source system 2 is completed within a 1/2 cycle, so that if there is a power interruption from the first alternating current power source system 1, the changeover operation to the second alternating current power source system 2 is performed without adversely affecting the load 5. In the figure, there are also shown a voltage transformer 61 for detecting the voltage at the first alternating current power source system 1, another voltage transformer 71 for detecting the voltage at the second alternating current power source system 2, and a switch controller 672 for receiving the outputs of the voltage transformers 61 and 71. The switch controller 672 controls the above-described first thyristor switch 6 and second thyristor switch 7 to selectively turn them on and off in response to the outputs of the voltage transformers 61 and 71.
As can be understood from the above discussion, in the selective power supply system or selective power receiving system, having these thyristor switches, a load current always flows through either the first thyristor switch 6 or the second thyristor switch 7. Accordingly, when a power loss attributed to an internal loss in the thyristors takes place, increased operating costs result from the generated power loss. Since the load current constantly flows through either the first thyristor switch 6 or the second thyristor switch 7, the thyristor switches 6 and 7 are subject to abnormal currents such as short-circuit currents and overload currents, and the current resistance rating of the thyristor elements forming the thyristor switch must be high enough to withstand transient abnormal currents for short periods of time. Moreover, thyristor switch and the thyristor elements forming the thyristor switch require a large cooling system to dissipate heat generated as a result of continuously flowing load currents and transient abnormal currents, leading to a bulky and costly design.